(1) Field of the Invention
The invention relates to an electromagnetic signal converter for an osteophone, comprising                a soft magnetic yoke,        an electrical coil arranged concentrically to the longitudinal axis of the yoke,        an elastically suspended, soft-magnetic armature which, viewed in the direction of the longitudinal axis of the yoke, is separated from the yoke by a working air gap and can move along the longitudinal axis of the yoke, and        a permanent magnet which is magnetized in the direction of the longitudinal axis of the yoke in order to generate a magnetic biasing voltage of the yoke and of the armature.        
The magnetic biasing voltage generates during the operation of the electromagnetic signal converter a current-proportional production of force on the armature and therefore an exact transfer of the electrical oscillations into mechanical oscillations. Without this magnetic biasing voltage the force and therefore the mechanical deflection would be proportional to the square of the current, which would result in a significant distortion by the frequency doubling and the suppression of the weak signals.
(2) Description of Related Art
Osteophones, as they are known from the prior art, convert electrical signals into mechanical oscillations and therefore function as oscillation generators or electromagnetic signal converters. This technology is used, among other things, in hearing aids and is especially suitable for persons with impairment of the outer ear and of the middle ear since in this case the sound cannot be mechanically transferred to the cochlea. However, osteophones can also be used in other hearing systems and communication systems where a transmission of sound through the air to the eardrum is not possible, for example, under water. Therefore, osteophones can be used in communication systems for divers. Osteophones can also be used in communication systems in which a transmission of sound through the air is basically possible but transmitted sound is hardly audible on account of surrounding noise such as in heavy industry (e.g. in steel mills).
The acoustical signal to be transferred to the person is recorded as a rule by a microphone (however, it could also be transferred as a radio signal), converted in an amplifier, prepared and forwarded as an electrical signal to the electromagnetic signal converter. In the signal converter the electrical signals are supplied to the coil which causes the armature to oscillate in a corresponding manner. The oscillator (osteophone) serving as armature contacts the skull bone, preferably the mastoid bone, wherein the acoustic signal is transmitted in the form of tactile oscillations while circumventing the middle ear directly into the inner ear where it is converted into a nerve stimulation in the cochlea.
These osteophones are usually built into a carrier object, for example, into a spectacle frame, a hair loop or into an external housing for wearing in a head covering.
The traditional construction of the signal converter has the disadvantage that the permanent magnet is constructed as an annular magnet, that is, it has the shape of a hollow cylinder which surrounds an annular coil and makes contact on a front side with a disk-shaped part of the yoke, the yoke plate, while on the other front side it faces the armature while maintaining an air gap, a so-called working air gap. This has the disadvantage that the magnetic flux excited by the magnetic flux of the permanent magnet as well as the magnetic flux excited by the coil use the same flux paths, namely, in the longitudinal direction through the yoke, in particular through a rod-shaped part of the yoke (yoke core), radially through the armature into the annular magnet, in the longitudinal direction through the annular magnet and again into the yoke, in particular radially through the yoke plate back into the yoke core. That means that even the coil flux must overcome the high magnetic resistance of the annular magnet. Therefore, in order to produce a certain magnetic change of flux through the coil a high, electrically excited flowthrough (large ampere windings) is required. This is equivalent to a high current or a high winding number, and in any case a high exciter output is needed for the coil, which again has as consequence a low service life of the battery of the osteophone.